This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Vaccinia virus poly(A) polymerase (VP55) is the only RNA polymerase known to translocate with respect to its RNA primer, raising a paradox: Which is more flexible: The polymerase or the primer? The protein has two known modes of action, namely the processive synthesis of 30 nt tails and, as a heterodimer with its processivity factor (VP39), the semi-processive synthesis of 200 ? 300 nt tails. Past, collaborative studies, have yielded crystal structures for VP39, and for the VP55-VP39 heterodimer. The most important, yet absent information is a structural understanding of the heterodimer-primer complex. By using the beam time of a colleague, a full-time crystallographer in the Gershon lab has progressed markedly in finding binding sites for short RNA segments on the polymerase surface. Although this is ground-breaking, original work for a poly(A) polymerase, 3 hours of beam time every 2 months provides inadequate progress in this NIH-funded work to ensure grant renewal. Beam time is the major bottleneck, hence the current request for individual, regular beam time for this crystallographic work.